The Science of Making Jam (Fruit Jelly)

If you have fruit trees in your garden you’re probably also swamped with fruit during certain times of the year. All of a sudden, you’ve got 5kg of figs and 20kg of apples. And not enough time to eat them all or enough place to store them properly.

This problem isn’t new of course and for centuries, if not longer, people have found ways to preserve these valuable fruits throughout the year. Making them into jams is one way to preserve them over time. By now we understand why making them into jam preserves them, but even before people knew where spoilage mouolds come from, jam making was a well known technique.

What is a fruit jam?

Jamming, the verb, means “to squeeze or pack tightly into a specified space”. This describes a fruit jam quite well: it is a lot of fruit jammed into a small space (a jar) with a bunch of added sugars.

In order to pack so many fruits together, you have to break down their structures completely. This is what gives jam a smooth, soft texture, without a lot of crunch or bite to it. It makes it into one homogeneous mass. The added sugars, as well as those naturally present in the fruit, make your jam sweet.

Another characteristic of a fruit jam is they way is flows. If you would simply puree fruits down, you end up with quite a liquid mass (especially if you blend fruits such as strawberries). A jam though doesn’t flow as easily, it is quite viscous and behaves like a gel.

The reason for this behavior is twofold. First of all, when you make a jam you boil off a lot of water which thickens up the jam and concentrates it even further. Second of all, pectin, which is naturally present in a lot of fruits, helps build that gel, a bit like gelatin does in a panna cotta!

Why jam is a way to preserve fruits

So why is this thicker mass of broken down fruits a good way to preserve fruits? Breaking down the fruits may sound counterproductive since a whole fruit has quite some mechanisms in place to help it stay fresh. The skins protect the inside, so if anything, fruit juices and blends are even more prone to spoilage than the fresh whole fruit. Once the skin is broken down, all the liquids and sugars (aka food for micro organisms) inside the fruit are freed up, easily accessible to spoilage micro organisms. The way jam solves this problem is by adding even more sugars to the fruit and getting rid of some of the water. The explanation for why this works lies in the concept of ‘water activity’.

Lowering the water activity

When discussing the shelf life of foods one term that comes back all the time is water activity. The water activity describes the amount of ‘available’ water in a food. Food with a high water activity (maximum value is 1,00) have a lot of available water. Fresh fruits have a high water activity.

Micro organisms that can spoil food also need a certain water activity to grow and survive. If the water activity is too low, the micro organisms cannot grow any more.

There are several ways to lower the water activity. First of all, you can get rid of some of the water. Secondly, you can add in other ingredients that will hold on to some of the water and make it less available. Sugar happens to be very good at that.

When you make jam you use both these strategies. You remove water during the boiling process and add a lot of sugar. The resulting water activity will be low enough to make it very unattractive for micro organisms to grow in.

On top of that, fruits tend to be slightly sour naturally. This low pH is another hurdle for micro organisms since most micro organisms, especially those that make you sick, will not be able to grow under these acidic conditions.

But I’ve seen moulds grow on my jam

Despite all these hurdles that we’ve thrown up against the micro organisms, there are still ways for it to spoil.

Once you’ve opened a bottle of jam inserted a dirty knife or spoon, micro organisms have been reintroduced. They won’t grow in the center of the jam, where it’s too acidic, dry and sugary for them. They can however, grow on the rims and sides of the jam and jar where there’s little bits of left over jam. On these little left overs condensation might form. This additional layer on the jam dilutes the jam and sugar content. All of a sudden micro organisms, especially yeasts and moulds, might be able to grow again. This is why you should store an opened pot of jam in the fridge.

forest fruit pie with linzertorte
Jan also works great as a filling for a pie!

How is jam made?

Since we need to break down and concentrate those fruits, making jam always starts by boiling fruits. The heat will break down the structure of the fruit. The cell walls are broken down and the moisture from inside the fruit is released. Other structures inside the fruit also break down, causing the whole fruit to soften. The sugar in the jam is added at the start of the boiling process. The sugar will dissolve in the water that comes out of the fruits.

By boiling the fruits not only does the texture change, the water content is also reduced. This is very important for making the jam keep good for a long period of time. By boiling of the liquid you’re increasing the concentration of sugar and fruit components. The jam is finished when it’s become thick enough so it doesn’t flow by itself anymore. Jam at this point will have pretty complex flow properties.

The role of pectin in jam

You will often see pectin being added to the fruits when making a jam. Pectin is a large complex polysaccharide (a carbohydrate) and has the ability to form gels. The large and complex structure of pectins can prevent other molecules (e.g. water) from moving around freely. As a result, the water is more or less kept in place and a gel is formed.

Some recipes for making jam will call for pectin and a lot of supermarket jams contain pectin as well. This is fine, but often you can well do without the additional pectin. As a matter of fact, various fruits (e.g. apples and berries) contain plenty pectin themselves to form this thicker gel-like structure.

Pectins seems to prefer a slightly acidic environment, this is when they are at their best with regards to gelling. Adding a little bit of acidity, e.g. lemon juice, can therefore improve the effectiveness of the pectins.

The structural role of sugar

Despite pectin’s gelling properties, these do not tend to be strong enough to gel cooked fruits. This is where the sugar comes into play as well. A sugar syrup with only little water is already thick and viscous. It will not flow as freely. The sugar holds on to the water molecules. This effect, combined with the pectin, is what makes the jam become nice and thick.

If you’ve ever made a jam and boiled it for too long. That is, it didn’t burn, but it became way too thick, there is no reason to worry either. Just add some more water and bring to the boil again. The structure of a sugar syrup is reversible so you can just try again. Even better, if you’re using the same fruit type, you can use a thermometer to determine when the jam is ready. The boiling point of a sugar solution depends on the concentration of sugar. So once you know which consistency you’re looking for, take care to note the temperature. Next time you’re making the jam you will only have to boil up to that temperature and the jam will be ready.

making jam: fig and grape jam in bowls

Sugar-free jam

Making sugar free jam is possible, however, it isn’t really jam anymore. Without the sugar the shelf life of the jam will be a lot shorter, probably days instead of months. Also, the flavour will be completely different as will the texture. In the supermarket you will therefore tend to see jam with less sugar and not without sugar. These sugar reduced jams still tend to contain more than 30% sugar. Also, they will contain additional ingredients to preserve the jam. For instance, they might contain potassium sorbate. Potassium sorbate inhibits the growth of yeasts and moulds and thus takes over some of that role of sugar.

homemade grape jam

Jam recipe

Yield: 2-3 pots
Prep Time: 10 minutes
Cook Time: 1 hour 30 minutes
Total Time: 1 hour 40 minutes

Making jam always consists of a very similar procedure, take fresh fruits and boil them with sugar. If you prefer a sweeter jam than the one below, just add some more sugar. Take care not to remove too much sugar though, as we discussed above, the jam might not keep as long anymore with less sugar.


  • 1 kg of fruits, skins removed and cut into smaller pieces
  • 500g of sugar

Pineapple; one of our favorites for sure, remove the skin and cut into smaller pieces Figs; use a spoon the remove the inside flesh from the skin Grapes; add the whole graps, but make sure to sieve the mixture a few minutes after you start boiling to remove the seeds and skins, the skins aren't too bad, but the seeds make the jam very unappealing. Fresh berries; just add them whole, if there are a lot of seeds, sieve the mixture, you can also use frozen ones

  • Pineapple; one of our favorites for sure, remove the skin and cut into smaller pieces
  • Figs; use a spoon the remove the inside flesh from the skin
  • Grapes; add the whole graps, but make sure to sieve the mixture a few minutes after you start boiling to remove the seeds and skins, the skins aren't too bad, but the seeds make the jam very unappealing.


  1. Cut the fruits into smaller pieces and place in a pot on the stove on a low heat.
  2. It's best not to add any water at this point. However, if you need to because your fruits are caramelizing on the bottom (getting brown), add a very thin layer of water. The amount of water will increase by itself while the fruits are breaking down and releasing their moisture.
  3. Once the fruit has broken down well enough, you can sieve the mixture to remove seeds or larger particles. Not all fruits require sieving though.
  4. At this point the individual fruit pieces should have broken down and become nice and soft.
  5. Add the sugar and keep on boiling on a low heat. Do not put it on a high heat, it will start browning and sticking to the bottom of the pan. Stir regularly.
  6. Your jam is ready when it stays firm once it's cooled down. Any easy way to test is to take a small bit out of the pan and leave it to cool to room temperature. If you measure the temperature at the moment it's done you will know the next time for how long to cook the jam.
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